The present invention relates to a method and apparatus for coding of digital video images such as video object planes (VOPs), and, in particular, to motion estimation and compensation techniques for interlaced digital video. A padding technique for extending the area of an interlaced coded reference VOP is also disclosed.
The invention is particularly suitable for use with various multimedia applications, and is compatible with the MPEG-4 Verification Model (VM) standard described in document ISO/IEC/JTC1/SC29/WG11 N1642, entitled "MPEG-4 Video Verification Model Version 7.0", April 1997, incorporated herein by reference. The MPEG-2 standard is a precursor to the MPEG-4 standard, and is described in document ISO/IEC 13818-2, entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.262," Mar. 25, 1994, incorporated herein by reference.
MPEG-4 is a new coding standard which provides a flexible framework and an open set of coding tools for communication, access, and manipulation of digital audio-visual data. These tools support a wide range of features. The flexible framework of MPEG-4 supports various combinations of coding tools and their corresponding functionalities for applications required by the computer, telecommunication, and entertainment (i.e., TV and film) industries, such as database browsing, information retrieval, and interactive communications.
MPEG-4 provides standardized core technologies allowing efficient storage, transmission and manipulation of video data in multimedia environments. MPEG-4 achieves efficient compression, object scalability, spatial and temporal scalability, and error resilience.
The MPEG-4 video VM coder/decoder (codec) is a block- and object-based hybrid coder with motion compensation. Texture is encoded with an 8.times.8 Discrete Cosine Transformation (DCT) utilizing overlapped block-motion compensation. Object shapes are represented as alpha maps and encoded using a Content-based Arithmetic Encoding (CAE) algorithm or a modified DCT coder, both using temporal prediction. The coder can handle sprites as they are known from computer graphics. Other coding methods, such as wavelet and sprite coding, may also be used for special applications.
Motion compensated texture coding is a well known approach for video coding, and can be modeled as a three-stage process. The first stage is signal processing which includes motion estimation and compensation (ME/MC) and a two-dimensional (2-D) spatial transformation. The objective of ME/MC and the spatial transformation is to take advantage of temporal and spatial correlations in a video sequence to optimize the rate-distortion performance of quantization and entropy coding under a complexity constraint. The most common technique for ME/MC has been block matching, and the most common spatial transformation has been the DCT.
However, special concerns arise for ME/MC of VOPs, particularly when the VOP is itself interlaced coded, and/or uses reference images which are interlaced coded. Moreover, for arbitrarily shaped VOPs which are interlaced coded, special attention must be paid to the area of the reference image used for motion prediction.
Accordingly, it would be desirable to have an efficient technique for ME/MC coding of a VOP which is itself interlaced coded, and/or uses reference images which are interlaced coded. The technique should provide differential encoding of the motion vectors of a block or macroblock of the VOP using motion vectors of neighboring blocks or macroblocks. A corresponding decoder should be provided. It would further be desirable to have an efficient technique for padding the area of a reference image for coding of interlaced VOPs. The present invention provides a system having the above and other advantages.